The present invention relates to a process for preparing a polyimide that has excellent thermal stability and processability in a molten state.
Polyimide obtained by reacting tetracarboxylic dianhydride and aromatic diamine has excellent mechanical strength, dimensional stability, high temperature stability, flame resistance and electrical insulative properties. Such a polyimide has been used in electrical and electronic appliances space and aeronautical instruments and transport machinery. It is expected that it will also be used in applications where high-temperature resistance is required.
A variety of polyimides exhibiting excellent properties have been developed. Some polyimides, however, have no distinct glass transition temperature, although they are excellent in high-temperature stability. As a result, when employed as molding materials, specific methods such as sinter molding must be used for processing.
Other polyimides which are excellent in processability have low glass transition temperatures and are soluble in halogenated hydrocarbons, which renders these polyimides unsatisfactory for use in applications which require high-temperature stability and solvent resistance. Thus, these polyimides have numerous advantages and disadvantages.
D. J. Progar et al. have previously discovered a polyimide that is excellent in mechanical strength, thermal characteristics, electrical properties and solvent resistance and exhibits high-temperature stability. The polyimide consists primarily of recurring units of the formula (I) (U.S. Pat. No. 4,065,345); ##STR1## Furthermore, D. J. Progar et al. have previously discovered a polyimide that consists primarily of recurring units of the formula (II) (Int. J. Adhesion and Adhesive, 4, No. 2, 79.about.86, Apr. 1984); ##STR2## The polyimide is a high-temperature stable resin having many good properties.
Polyimides exhibit excellent flowability and good processability. The fused resin, however, gradually exhibits decreased flowability which has an adverse effect on processability when the polyimide is kept at high temperatures for a long period of time, for example, long residence at high temperature in a cylinder used in the injection molding.
Therefore, it is desirable to develop a polyimide which exhibits good flowability at lower temperatures and stable flowability for a long period of time during processing steps.